If there is a wish that heat transfer coefficient is increased to enhance heat exchange effectiveness, in case of utilizing a heat exchanger by which interchange of heat (heat exchange) is made between a high temperature fluid and a low temperature fluid, a plate-type heat exchanger has conventionally been used widely. The plate-type heat exchanger has a structure in which a plurality of heat transfer members having a plate-shape are placed parallelly one upon another at prescribed intervals so as to form passages, which are separated by means of the respective heat transfer member. The high temperature fluid and the low temperature fluid flow alternately in the above-mentioned passages to make heat exchange through the respective heat transfer members.
In the conventional heat exchanger having the above-described structure, a gasket formed of elastic material is placed between the adjacent heat transfer members to keep the distance of the space between them constant and divide the space into passages for the fluids. However, an increased pressure of the heat exchange fluids passing through the space between the heat transfer members may cause occurrence of deformation of the gasket, thus making it impossible to separate properly the heat exchange fluids from each other and leading to improper change in distance between the heat transfer members, resulting in difficulty in effective heat exchange. Consequently, use of the heat exchange fluids is limited only within a pressure range that the gasket can withstand.
In view of such circumstances, the other plate-type heat exchanger having a structure in which the heat transfer members formed of metallic sheet and placed at a predetermined interval are directly connected at their edges through a brazed joint, without using any gasket, so as to combine the heat transfer members together, while forming passage portions on the opposite sides of the respective heat transfer member, has recently been put into practical use.
The conventional heat exchangers have the above-described structures. The heat exchanger in which the heat transfer members are brazed, can withstand a higher pressure of fluid than the heat exchanger utilizing the gasket. However, the brazing metal utilized to connect the heat transfer members to each other has a lower strength than the portion of the metallic sheet, due to the connection mechanism and property of brazed material. As a result, such a heat exchanger has a structure in which the strength in the joint portions of the heat transfer members is considerably deteriorated. When there is a large difference in pressure between the heat exchange fluids flowing on the opposite surfaces side of the heat transfer member and the high pressure heat exchange fluid passes the space between the heat transfer members connected to each other through the brazed joint, the brazed joint portion may break to cause the fluid to leak from the high pressure side to the low pressure side. Accordingly, the high pressure heat exchange fluid has not still been fully utilized, thus causing a problem.
With respect to connection of the metallic sheets, in which such a tight sealing property is regarded as important, it is preferable to apply a seam welding method, which provides the connected portion with a sufficient strength to enable the connected portion to withstand continuous and high pressure. However, the heat exchanger has the structure in which the metallic sheets for the heat transfer members are arranged in a line in the extremely short distance. As a result, when the conventional seam welding apparatus is used, a certain portion(s) of thereof may interfere with the metallic sheet, which is placed in parallel with the other metallic sheet to be welded. It has therefore been difficult to locate welding electrodes so as to place the metallic sheet to be welded between the welding electrodes, thus making it impossible to carry out the seam welding.
An object of the present invention is therefore to provide a seam welding apparatus, which enables, even when metallic sheets are arranged in a line in a short distance, welding electrodes to come into contact only with the metallic sheet to be welded, so as to provide a reliable welding of the metallic sheets to form connected portions having high strength.